Dosing belt weighers are equipped with a load cell for producing a total belt load signal. Such weighers are also equipped with a sensor device for producing a belt r.p.m. signals. Both signals are supplied to a taring device forming part of a closed loop control circuit. The taring device corrects the total belt load signal in accordance with correction values that represent localized belt influencing data or information allocated to a particular belt section.
Dosing belt weighers are special conveyor belts equipped with means for dosing bulk material to be supplied in precisely adjustable conveyed quantities to a processing operation, for example, a chemical process. For ascertaining the conveyed quantity, the load applied to the conveyor belt and the speed of the conveyor belt are measured to provide respective signals. The product of the net belt loading and the belt speed corresponds to the conveyed quantity which must be dosed and which thus requires to be precisely controlled by means of a closed loop circuit. The total belt load is measured to provide a gross or total belt load signal which comprises several signal components. A first signal component of the total belt load signal represents the load caused by the weight of the bulk material. A second signal component represents the weight of the belt itself. A third signal component represents the forces due to belt-stiffness causing a belt influence. By taking into account a complete belt revolution it is possible to obtain a mean belt influence value and to use this mean belt influence value for correcting the total belt load signal. As a result of such correction, one obtains a net value of the weight load which corresponds to the weight of the bulk material being transported on the belt during each belt revolution.
For several modern production methods, especially in the chemical industry, it is required that the supply of dosed quantities of bulk material is precisely constant. In other words, the conveyed quantity of bulk material dropping off the discharge edge of the conveyor belt, must be constant. However, such a precise constancy is not achievable by a correction that relates to full belt revolutions because the belt influence caused by influences other than the bulk material weight, is only constant on average while being subject to substantial fluctuations during one full belt revolution. In order to take the belt influence into account when making the tare correction, it is known to ascertain the components which contribute to the belt influence when no bulk material is being conveyed by the conveyor belt and to allocate the results to individual belt sections while these results are being stored in a memory. During the subsequent weighing operation, the stored correction results are retrieved from the memory in the proper sequence for a correct allocation to the respective belt section, whereby these correction values are subtracted from the measured or actual total belt load signals so that one obtains for each belt section a net belt load signal. In other words, the net belt load signal is obtained for each belt section by deducting the local belt influence signal from the total belt load signal. However, this known method does not lead to the desired dosing precision because the characteristic of the belt influencing function is ambiguous since it depends substantially on several variables, such as the type of operation of the dosing belt weigher, the duration of the belt run, and the belt temperature. Thus, the belt influence depends on slowly time variable influences which cannot be ascertained and compensated by the known method.